Mechanical Performance and Failure Characteristics of Variable-Section Deep Cement Mixing Columns in Improved Composite Foundation

Conventional deep cement mixing (DCM) columns commonly experience performance constraints and site-specific challenges arising from heterogeneous geological and loading conditions. This study investigates the vertical stress distribution, settlement behavior, and failure mechanisms of Variable-Section Deep Cement Mixing (VSDCM) columns through a series of finite element modeling. A comparative assessment is also conducted with two uniform-diameter columns of 0.5 m and 0.8 m. It is evident that the VSDCM columns possess 90% of the bearing capacity of the corresponding large-diameter columns. It exhibits a relative settlement 4–5 times smaller than that of the small-diameter column composite foundation, indicating a dominant role of enlarged head in stress redistribution and load sharing within the composite foundation. The stress arch exhibits a vertical influence range of approximately 0–0.4 m, within which load redistribution is significant. The VSDCM column encounter two stress peaks due to its variable cross-section, triggering failure at both, with the small-diameter section beneath the enlarged head being the most critical zone. The reduced material demands of the VSDCM column results in higher engineering economy, supporting its applicability as a sustainable and cost-effective ground improvement technique.